System and method for reducing dynamic range and improving linearity in an amplication system

ABSTRACT

An amplification system and method is provided that reduces peaks associated with an input signal, and provides correction, in one or more spectral bands, to signal distortion and out-of-band emissions that result from the peak reduction. The correction signal that removes signal distortion and OOB emissions associated with the peak reduction can be calculated or electronically derived. The correction signal can be combined with the peak reduced signal prior to or after amplification of the peak reduced input signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/625,367, filed Jul. 23, 2003, entitled SYSTEM AND METHOD FOR REDUCINGDYNAMIC RANGE AND IMPROVING LINEARITY IN AN Amplication System, andassigned to the same assignee as the present application. Thisapplication is related to U.S. patent application Ser. No. 10/625,378,filed Jul. 23, 2003, entitled DIGITAL CROSS CANCELLATION SYSTEM, andassigned to the same assignee as the present application.

TECHNICAL FIELD

The present invention relates generally to electronic devices, and moreparticularly to a system and method for reducing dynamic range andimproving linearity in an amplification system.

BACKGROUND OF THE INVENTION

RF power amplifiers used for wireless communication transmitters, withspectrally efficient modulation formats, require high linearity topreserve modulation accuracy and to limit spectral regrowth. Typically,a linear amplifier, Class-A type, Class-AB type or Class-B is employedto faithfully reproduce input signals and to limit the amplifier outputwithin a strict emissions mask. Linear amplifiers are capable ofelectrical (DC power in to RF power out or DC-RF) efficiencies 50% orgreater when operated at saturation. However, they are generally notoperated at high efficiency due to the need to provide high linearity.For constant envelope waveforms, linear amplifiers are often operatedbelow saturation to provide for operation in their linear regime. Timevarying envelopes present an additional challenge. The general solutionis to amplify the peaks of the waveform near saturation, resulting inthe average power of the waveform being amplified at a level wellbacked-off from saturation. The back-off level, also referred to asoutput power back-off (OPBO), determines the electrical efficiency of alinear amplifier.

For example, the efficiency of a Class-A type amplifier decreases withoutput power relative to its peak value (EFF=P_(OUT)/P_(PEAK)). Theefficiency of Class-B type amplifiers also decreases with output powerrelative to its peak value (EFF=(P_(OUT)/P_(PEAK))^(1/2)). Class-AB typeamplifiers have output power variations intermediate between thesevalues. Thus, there is customarily an inherent tradeoff betweenlinearity and efficiency in amplifier designs.

Modern transmitters for applications such as cellular, personal, andsatellite communications employ digital modulation techniques such asquadrature phase-shift keying (QPSK) in combination with code divisionmultiple access (CDMA) communication. Shaping of the data pulsesmitigates out-of-band emissions from occurring into adjacent channelsbut produces time-varying envelopes. In addition to amplifyingindividual waveforms with time varying envelopes, many transmitters(especially in base stations) are being configured to amplify multiplecarriers. Multi-carrier signals have a wide distribution of power levelsresulting in a large peak-to-average ratio (PAR). Therefore, theoperation of the linear amplifiers in these types of signals is veryinefficient, since the amplifiers must have their supply voltage sizedto handle the large peak voltages even though the signals are muchsmaller a substantial portion of the time. Additionally, the size andcost of the power amplifier is generally proportional to the requiredpeak output power of the amplifier. Techniques that limit out-of-band(OOB) emissions while the amplifier operates at or near saturation arehighly desirable.

Wideband Code Division Multiple Access (WCDMA), Orthogonal FrequencyDivision Multiplexing (OFDM), and multi-carrier versions of GlobalStandard for Mobile Communication (GSM) and Code Division MultipleAccess 2000 (CDMA 2000) are wireless standards and application growingin use. Each requires amplification of a waveform with high PAR levelsabove 10 dB in some cases. The sparse amount of spectrum allocated toterrestrial wireless communication requires that transmissions minimizeout-of-band (OOB) emissions to minimize the interference environment. Alinear amplifier used to amplify a waveform with a PAR of 10 dB or moreprovides only 5–10% DC-RF efficiency. The peak output power for theamplifier is sized by the peak waveform. The cost of the amplifierscales with its, peak power. Several other circuit costs including heatsinks and DC-DC power supplies scale inversely to peak power anddissipated heat (which results from the electrical inefficiency).Related base station costs of AC-DC power supplies, back-up batteries,cooling, and circuit breakers also scale inversely with efficiency asdoes the electrical operating costs. Clearly, improving DC-RF efficiencyis a major cost saver both for manufacture and operation.

Many modern digital communications systems transmit complex waveformsconsisting of multiple carriers, multiple code channels, or othersignals that give rise to large, infrequent peaks in signal power. Thesesignals, while rich in information content, are costly to transmit interms of hardware and electrical consumption. Any scheme that reducesthe size of the peaks without introducing substantial levels of error isdesirable. Most modern day communication standards strictly limit theamount of signal distortion and OOB emissions that can occur in a signaltransmission. There are a variety of schemes to clip a signal whichresult in substantial amounts of signal distortion and/or OOB emissions.The strict regulation of OOB emissions is often the limiting factor inthe degree to which peak signals can be limited by clipping.

Linearization techniques generally improve wanted signal distortion andreduce OOB emissions. Some linearization techniques, such as digitalpre-distortion and versions of digital cross cancellation require apriori information on the mechanisms that cause distortion and OOBemissions. Intentional clipping of a signal results in OOB emissionsthat can be readily predicted. Most linearization techniques operateover a limited bandwidth but these can be adapted to work in severalparallel channels with greater efficacy.

SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is intendedneither to identify key or critical elements of the invention nordelineate the scope of the invention. Its sole purpose is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented later.

The present invention relates to an amplification system and method thatreduces peaks associated with an input signal and provides correction tothe reduced peak signal associated with signal distortion and/or OOBemissions that result from the peak reduction. This provides a finalamplified output signal substantially free of signal distortion and/orOOB emissions. The correction signal can be combined with the peakreduced signal prior to (e.g., pre-distortion) or after amplification ofthe peak reduced input signal (e.g., digital cross cancellation). Theinput signal can be clipped, for example, via a clipping filter toreduced peaks associated with the input signal. The correction signalcan be calculated that removes signal distortion and/or OOB emissionsassociated with the clipped input signal. An anti-peak signal can becombined with the input signal to reduce peaks associated with the inputsignal. The correction signal that is combined with the peak reducedinput signal after final amplification is one that appropriately cancelsthe “anti-peak” signal. The anti-peak signal can be combined with theinput signal prior to or after digital-to-analog conversion of the inputsignal.

In one aspect of the invention, the peak reduced input signal isseparated into a plurality of sub-bands by a channelizer. Each sub-bandis provided with an associated modification component that can modify atleast one of gain, phase and offset of the sub-band signal to mitigatesignal distortion and OOB emissions associated with one or moresub-bands prior to final amplification. The plurality of sub-bands canthen be aggregated to provide an aggregated signal. The aggregation canoccur prior to or after digital-to-analog conversion. This technique canbe employed alone as an improved (e.g., wider bandwidth) pre-distortiontechnique, as a pre-distortion component with the correction signalamplification system or as part of the correction signal amplificationsystem.

To the accomplishment of the foregoing and related ends, certainillustrative aspects of the invention are described herein in connectionwith the following description and the annexed drawings. These aspectsare indicative, however, of but a few of the various ways in which theprinciples of the invention may be employed and the present invention isintended to include all such aspects and their equivalents. Otheradvantages and novel features of the invention will become apparent fromthe following detailed description of the invention when considered inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic block diagram of an amplification systemin accordance with an aspect of the present invention.

FIG. 2 illustrates a schematic block diagram of an amplification systemthat combines a correction signal with an amplified peaked reduced inputsignal in accordance with an aspect of the present invention.

FIG. 3 illustrates a schematic block-diagram of an amplification systemthat combines a correction signal with a peaked reduced or clipped inputsignal prior to amplification in accordance with an aspect of thepresent invention.

FIG. 4 illustrates a schematic block diagram of an amplification systemthat separates an input signal into a plurality of transmissionsub-bands in accordance with an aspect of the present invention.

FIG. 5 illustrates a schematic block diagram of an amplification systemthat combines a digital anti-peak signal with a digital input signal inaccordance with an aspect of the present invention.

FIG. 6 illustrates a schematic block diagram of an amplification systemthat combines an analog anti-peak signal with an analog input signal inaccordance with an aspect of the present invention.

FIG. 7 illustrates a schematic block diagram of an amplification systemthat employs a digital cross-cancellation technique in accordance withan aspect of the present invention.

FIG. 8 illustrates a block diagram of a communication system inaccordance with an aspect of the present invention.

FIG. 9 illustrates a methodology for amplifying an input signal inaccordance with an aspect of the present invention.

FIG. 10 illustrates another methodology for amplifying an input signalin accordance with an aspect of the present invention.

DETAILED DESCRIPTION OF INVENTION

The present invention relates to an amplification system and method thatreduces distortion and/or OOB emissions in amplifier systems such asthose resulting from the clipping of peaks associated with an inputsignal. The system and method provide one or more corrections to thereduced peak signal associated with signal distortion and OOB emissionsthat result from the peak reduction. Therefore, smaller (less powercapacity) and less costly power amplifiers can be employed to achievesimilar performance. A second result is improved amplifier systemefficiency as compared to amplifier systems with much larger lessefficient power amplifiers. The present invention can be employed inwireless standards such as WCDMA, OFDM, multi-carrier versions of GSMand CDMA 2000 and other wireless standards and applications.

FIG. 1 illustrates an amplification system 10 in accordance with anaspect of the present invention. The amplification system 10 includes apeak reduction component 12 that receives an input signal and generatesa peak reduced input signal. The peak reduction component can clip peaksfrom the input signal by employing a clipping filter or the like toremove large peaks from the input signal. Alternatively, the peakreduction component 12 can be a peak reduction shaping algorithm.Furthermore, the peak reduction component can add signals (e.g.,anti-peaking signals, anti-distortion signals) to the input signal toreduce peaks and distortions associated with the input signal. The peakreduction component can also perform pre-distortion on the peak reducedsignal to mitigate errors to signal amplitude or phase and OOB emissionscaused by the peak reduction. The above modifications can be performedindividually, in combination or with other modification techniques toproduce a peak reduced input signal that is optimal with respect toamplifier linearity, efficiency and power consumption. The choice ofclipping option depends on the balance of allowable distortion in thewanted signal (e.g., error vector magnitude, EVM) with constraints forOOB emissions. The OOB emissions that limit many clipping schemes, andto some extent, the distortion of the wanted signals, are cancelled by adedicated subsystem before the signal is transmitted.

The peak reduced input signal is transmitted to a digital-to-analogconverter (DAC) 14. The DAC 14 converts the peak reduced input signalfrom the digital domain to the analog domain. The DAC can be a deltasigma modulated DAC (e.g., 1-bit DAC) to perform a digital-to-analogconversion directly to radio transmission frequencies. A one-bitconverter provides analog conversion with extremely high linearity (lowdistortion). Prior to digital-to analog conversion, the peak reducedinput signal can be provided to a pre-distortion component 13 to removeat least a portion of the signal distortion and/or OOB emissions causedby the peak reduction. Employing pre-distortion reduces the remainingdistortion so that components that are smaller, less costly and consumeless power can be employed. The output of the DAC 14 is then provided toan amplifier system 16.

The amplifier system 16 includes a power amplifier 24 for amplificationof the peak reduced input signal. The power amplifier 24 can be a linearamplifier (e.g., Class-A, Class-AB, Class-B) or, for some classes ofinput signal, it can be a non-linear type amplifier (e.g., Class-C,Class-D, Class-E, Class-F) based on desired performance, acceptableefficiency and acceptable OOB emissions. The limit for WCDMA and manyother wireless systems is the strict emissions mask in adjacent andnearby spectral channels which limits the amount of OOB that can betransmitted. The present invention illustrated in FIG. 1 is estimated tobe able to correct 20–40 dB of OOB emissions. Thus, the OOB induced byclipping the wanted signal should be corrected by an amount to assurethe OOB meets the emission mask of the relevant standard.

For most amplifiers, there is roughly a dB for dB savings in size andcost of the selected amplifier with the peak-to-average ratio (PAR)reduction of the amplification system. Therefore, the present inventionallows for employment of a power amplifier that is smaller (less power).Efficiency of linear amplifiers (class A, A/B, B) which are prevalent inwireless systems is normally proportional to the PAR (class A degradesin efficiency about dB for dB of PAR, class B one-half dB per dB, andclass A/B in-between), and thus the overall efficiency can besignificantly improved with peak reduction. A class A/B amplifiertransmitting a 10-dB PAR signal has total efficiency below 10% becausemost of the signals are extremely backed-off. The same transmitter canbe made with double the efficiency at about 5 dB PAR employing thepresent invention. The low frequency of occurrence of the peaks enablesthe present invention to aggressively clip peaks without significantdegradation of EVM. There can be dramatic degradation of OOB emissionsresulting from clipping, which the present invention corrects prior tofinal transmission. The increase in efficiency is critical in sizing thepower handling, backup battery, and cooling equipment at a base station.The cost savings for these related systems can be comparable to theprice of the transmitter.

A correction signal generator 20 provides a correction signal to correctfor signal distortion and OOB emissions caused by the peak reduction ofthe input signal. The signal distortion and OOB emissions can bemathematically computed based on the peak reduction that is performed.Therefore, a desired correction signal can be computed in real-time oroff-line and programmed into the correction signal generator 20 (e.g.,via a look-up table, a mathematical algorithm). The correction signal istransmitted to a second DAC 22. The second DAC 22 can also be a deltasigma modulated DAC to perform a digital-to-analog conversion directlyto radio transmission frequencies of the correction signal.

The analog correction signal is then provided to the amplifier system16. The analog correction signal can be combined with the peak reducedinput signal to mitigate signal distortion and OOB emissions prior toamplification by the power amplifier. Alternatively, the analogcorrection signal can be amplified and combined with the peak reducedinput signal after amplification to mitigate signal distortion and OOBemissions in addition to distortions associated with amplification bythe power amplifier. The output of the amplifier system 16 is thenprovided to an optional band pass filter 18, which filters out anyremaining unwanted signals outside the desired transmission band toprovide a final output signal substantially free of signal distortionand OOB emissions.

In one aspect of the invention, one or both of the first DAC 14 and thesecond DAC 22 are delta sigma modulated DACs. Delta Sigma modulation isa technique used to generate a coarse estimate of a signal using a smallnumber of quantization levels and a very high sampling rate. The smallnumber (two for a one-bit quantizer) of signal levels introduces“quantization” noise into the system. The effect of oversampling and theuse of an integrator feedback-loop in delta-sigma modulation areeffective in shifting noise to out-of-band frequencies. The noiseshifting properties and introduction of quantization error enablesefficient use of subsequent filtering stages to remove noise and producea more precise representation of the input at a much higher frequency.The delta sigma DACs can be employed to upconvert the input signaldirectly to radio transmission frequencies, such that further frequencyconversion of the signals via conventional analog mixers is notrequired. The radio transmission frequencies can be in radio frequency(RF) ranges (e.g., megahertz range) or in microwave frequency ranges(e.g., gigahertz range).

The correction signal can also be used purely for linearization in theabsence of clipping or peak reduction. This may be desirable as DAClimitations in dynamic range over wide bandwidths can limit digitallinearization techniques. The present invention separates the paths ofwanted signal and correction signal allowing bandwidth and dynamic rangerequirements to be allocated over two or more DACs.

FIG. 2 illustrates an amplification system 40 that employs a version ofdigital cross cancellation, combining a correction signal with anamplified peaked reduced input signal in accordance with an aspect ofthe present invention. The amplification system 40 includes a clipfilter 42 that performs a clipping process on an input signal. Theclipping process reduces the peaks of the input signal to reduce thepeak-to-average ratio of the input signal provided to a power amplifier50. This allows the power amplifier 50 to output a large average powerassociated with amplification of the peak reduced input signal. Theclipping process can be a soft or hard clipping process. Additionally,the clipping filter 42 can perform a fixed or shape limiting algorithmto reduced the peaks and PAR associated with the input signal. Theclipping process results in unwanted OOB emissions, spectraldistortions, spectral splatter and spectrum spreading. The unwantedcharacteristics associated with clipping need to be mitigated to conformto most wireless communication standards (e.g., WCDMA, OFDM, GSM).

A digital component (not shown) such as a digital signal processor,provides the input signal, a control signal for controlling the clippingassociated with the clipping filter and a correction signal to corrector mitigate OOB emissions caused by the clipping. The clipping filter 42provides a clipped input signal to a delta sigma modulator 44 along aninput path. The delta-sigma modulator 44 is coupled to a DAC 46 that iscoupled to a band pass filter 48. The delta-sigma modulator 44, the DAC46 and the band pass filter 48 cooperate to perform a digital-to-analogconversion directly to radio transmission frequencies. The DAC 46 can bea multi-bit converter or a one-bit converter that provides analogconversion with extremely high linearity (low distortion). The output ofthe band pass filter 48 is then provided to the input terminal of thepower amplifier 50 for amplification. The power amplifier 50 can be alinear amplifier (e.g., Class-A, Class-AB, Class-B) or, for some classesof input signal, it can be a non-linear type amplifier (e.g., Class-C,Class-D, Class-E, Class-F) based on desired performance, acceptableefficiency and acceptable OOB emissions. Additional frequency conversioncomponents can be employed as needed.

The digital component (not shown) provides the correction signal along acorrection path to a delta sigma modulator 56. The delta-sigma modulator56 is coupled to a DAC 58, which is coupled to a band pass filter 60.The correction signal can be of substantially lower power compared tothe wanted signal (e.g., typically 10–30 dB below the wanted signallevel, prior to amplification by the power amplifier). The lower powerlevel of the correction signal allows DAC 58 to be a lower dynamic rangeDAC than DAC 46. It can also have a wider bandwidth than DAC 46. Thedelta-sigma modulator 56, the DAC 58 and the band pass filter 60cooperate to perform a digital-to-analog conversion directly to radiotransmission frequencies. The DAC 58 can be a multi-bit converter or aone-bit converter that provides analog conversion with extremely highlinearity (low distortion). The correction signal can contain terms tosubstantially eliminate OOB emission resulting from clipping and OOBemissions resulting from the nominal amplification of the clippedsignal. The output of the band pass filter 60 is then provided to theinput terminal of a correction amplifier 62 for amplification. Thecorrection amplifier 62 should be substantially linear to provide anaccurate analog correction signal to cancel signal distortion and OOBemissions. The corrections signal will normally be 10–30 dB lower powerthan the wanted signal and does not require a large amplifier.Additional frequency conversion components can be employed as needed.

For example, a class A, A/B amplifier that is well backed-off can beemployed. The linearization of the cancellation amplifier 62 could be apre-distortion system (analog or digital; the latter requiringdigitizing and re-converting the signal, a feedforward loop or someother linearization technique. The size of the cancellation amplifier 62is dependent on the required correction level (i.e., OOB emissions,signal distortions). Therefore, pre-distortion can be employed with theamplification system 40 to reduce the size of the correction amplifier62.

The correction amplifier 62 provides an amplified analog correctionsignal that can be combined with an analog output signal of the poweramplifier 50 to mitigate OOB emissions and signal distortion of theanalog output signal as a result of the clipping by the clipping filter42. The analog correction signal of the cancellation amplifier 62 andthe analog output signal of the power amplifier 50 are combined at asummer or coupler 52. It may be necessary to include either digital oranalog delay components to synchronize the correction signal and outputsignal based on a particular implementation. The output of the summer 52is then provided to an optional band pass filter 54, which filters outany remaining unwanted signals outside the desired transmission band.

FIG. 3 illustrates an amplification system that combines a correctionsignal with a peaked reduced or clipped input signal prior toamplification in accordance with an aspect of the present invention. Theamplification system 70 includes a clip filter 72 that performs aclipping process on an input signal. The clip filter 72 can be similarto the clip filter described in FIG. 2. The clipping process reduces thepeaks of the input signal to reduce the PAR of the input signal providedto a power amplifier 78. This allows the power amplifier 78 to output alarge average power associated with amplification of the clipped inputsignal. The clipping process can be performed via soft clipping, hardclipping, and/or a fixed or shape limiting algorithm to reduced thepeaks and PAR associated with the input signal. The clipping process canresult in unwanted OOB emissions, spectral distortions, spectralsplatter and spectrum spreading.

A digital component (not shown) such as a digital signal processor,provides the input signal, a control signal for controlling the clippingassociated with the clipping filter and a correction signal to corrector mitigate OOB emissions caused by the clipping. The clipping filter 72provides the clipped input signal to a DAC 74 (e.g., delta sigmamodulated DAC) along an input path. The DAC 74 performs adigital-to-analog conversion of the clipped input signal, which can beconverted directly to radio transmission frequencies, to produce ananalog clipped input signal. The DAC 74 can be a multi-bit converter ora one-bit converter. The output of the DAC 74 is then provided to asummer or coupler 76.

The digital component (not shown) provides the correction signal to aDAC 82 along a correction path, which converts the correction signalfrom the digital domain to the analog domain to produce an analogcorrection signal. The DAC 82 can be a multi-bit converter or a one-bitconverter. The output of the DAC 82 is also provided to the summer orcoupler 76, which combines the analog clipped input signal with theanalog correction signal. The correction signal can be of substantiallylower power compared to the wanted signal (e.g., typically 10–30 dBbelow the wanted signal level (prior to amplification by the poweramplifier)). The DAC 82 can be wider band and lower dynamic range thanDAC 74. The analog correction signal mitigates OOB emissions and signaldistortion without the need for employing a cancellation amplifier. Theoutput of the summer 76 is provided to the input of the power amplifier78 for amplification. The output of the amplifier 78 is then provided toan optional band pass filter 80, which filters out any remainingunwanted signals outside the desired transmission band.

In another aspect of the present invention, at least one of the wantedsignal, adjacent channels, and nearby spectral bands are decomposed intosmall frequency slices or sub-bands, each of which is modified inamplitude and phase and given an offset. When these modified signals aredigitized and amplified they cancel the OOB emissions resulting from theclipping. These modified slices are either aggregated before of afterthe digital-to-analog converter (DAC) operation, depending on theavailable DAC bandwidth, and amplified to final power levels. Thistechnique can be applied to linearize the entire transmitter chain overwide bandwidths if distortions are predictable. This aspect of thepresent invention allows the correction terms to tailored to thatportion of the spectrum and avoids the problem of digital pre-distortionwhere a single DAC supplies the bandwidth for the correction terms anddynamic range for the wanted signal and emission mask. An optionalfeedback loop digitizes and channelizes a sample of the output to enableadaptive improvement of the gain and phase modifications.

FIG. 4 illustrates an amplification system 100 that separates an inputsignal into a plurality of transmission sub-bands in accordance with anaspect of the present invention. The amplification system 100 includesan optional clip filter 102 that removes peaks associated with an inputsignal and provides a clipped input signal to a channelizer 104. Thechannelizer 104 separates the clipped input signal, adjacent spectralchannels, and nearby spectral bands into a plurality of sub-bands thatcan be modified separately to remove OOB emissions and signaldistortions associated with each sub-band. A modification component 106is associated with each sub-band. The associated modification component106 modifies the gain and phase of the associated sub-band to removedistortions caused by the clipping of the input signal. Optionally, anoffset signal is added to each sub-band. The signal modifications arecomputed to cancel the OOB emissions and signal distortion that will bepresent after final amplification. This amplification system willprovide optimized linearization and/or OOB emission reduction over eachportion of the spectrum. The optimized linearization will occur if theclip filter 102 is removed.

A summer 108 recombines the sub-bands into an aggregated or recombinedinput signal. Alternatively, the sub-bands can be combined later in thesignal chain (e.g., after digital-to-analog conversion). Optimizinglater in the signal chain requires additional DACs but allows each DACto be optimized for bandwidth, dynamic range, and other performanceparameters. The aggregated input signal is provided to a DAC 110 thatconverts the aggregated input signal from a digital signal to an analogaggregated input signal. The analog aggregated input signal is thenprovided to an optional surface acoustic wave (SAW) filter 112. Theanalog aggregated signal is then provided to a power amplifier 114 forfinal amplification. An optional ADC and digital channelizer can beprovided to digitize a sample of the output and compare the sample ofthe output to the wanted signals to adaptively improve the gain, phase,and offset terms for each sub-band. Corrections signals can be providedto the signal prior to amplification and/or after amplification tofacilitate cancellation of signal distortion and OOB emissions.

The channelizer 104, the modification components 106 and the aggregationcomponent 108 can be employed as a pre-distortion component for avariety of different amplification systems. Additionally, thechannelizer 104, the modification components 106 and the aggregationcomponent 108 can be employed as a pre-distortion component in anamplification system such as that illustrated in FIG. 1. Furthermore,the modification component 106 can provide the desired correctionsignals to the individual sub-bands in the form of or in addition to thegain, phase and offset adjustments.

In another aspect of the present invention, an additional signal that isdesigned to cancel the peaks of the wanted signals is added to thewanted signals. This signal can be filtered or cancelled prior totransmission so as not to cause errors to the intended receiver or otherreceivers in the area. In some cases (e.g., standards based on CDMA) theadded signal is in the transmission band of the wanted signal but isorthogonal. The additional signal(s) may be added prior to and/or afterdigital-to-analog conversion occurs.

FIG. 5 illustrates an amplification system 120 that combines a digitalanti-peak signal with a digital input signal in accordance with anaspect of the present invention. The amplification system 120 includesan anti-peak signal component 122 that combines an anti-peak signal withan input signal to reduce peaks associated with an input signal, and tomitigate signal distortion and OOB emissions associated with either thereduction of the peaks or non-linearities in the signal chain. Thecombining of the input signal with the anti-peak signal reduces thepeaks of the input signal and the PAR of the input signal provided to apower amplifier 130.

The peak reduced input signal is transmitted to a delta sigma modulator124 along an input path. The delta-sigma modulator 124 is coupled to aDAC 126, which is coupled to a band pass filter 128. The delta-sigmamodulator 124, the DAC 126 and the band pass filter 128 cooperate toperform a digital-to-analog conversion directly to radio transmissionfrequencies. The DAC 126 can be a multi-bit converter or a one-bitconverter that provides analog conversion with extremely high linearity(low distortion). The output of the band pass filter 128 is thenprovided to the input terminal of the power amplifier 130 foramplification to provide an amplified peak reduced output signal. Thepower amplifier 130 can be a linear amplifier (e.g., Class-A, Class-AB,Class-B) or, for some classes of input signal, it can be a non-lineartype amplifier (e.g., Class-C, Class-D, Class-E, Class-F) based ondesired performance acceptable efficiency and acceptable OOB emissions.

A peak signal is provided to a delta sigma modulator 136 along acorrection path. The peak signal is a substantial inversion of theanti-peak signal and removes the anti-peak signal from the finalamplified output. However, if the anti-peak signal can be removed viafiltering, for example, by employing an orthogonal signal or output ofband signal with the peak reduced input signal, and the signal lossesare acceptable, addition of the peak signal may not be desired. The peaksignal may also contain terms to reduce OOB emissions and/or reducewanted signal distortion.

The delta-sigma modulator 136 is coupled to a DAC 138, which is coupledto a band pass filter 140. The delta-sigma modulator 136, the DAC 138and the band pass filter 140 cooperate to perform a digital-to-analogconversion directly to radio transmission frequencies. The DAC 138 canbe a multi-bit converter or a one-bit converter. The output of the bandpass filter 140 is then provided to the input terminal of a peakamplifier 142 for amplification. The peak amplifier 142 provides theamplified peak signal to a summer or coupler 132 to be aggregated withthe output signal from the power amplifier 130 to remove the anti-peaksignal from the final output. The output of the summer 132 is thenprovided to an optional band pass filter 134, which filters out anyremaining unwanted signals outside the desired transmission band. It maybe necessary to utilize digital or analog delay elements to synchronizethe peak signal and output signal based on a particular implementation.

FIG. 6 illustrates an amplification system that combines an anti-peaksignal with an input signal prior to amplification in accordance with anaspect of the present invention. This variant of the present inventioncan be used to introduce an out-of-band anti-peaking signal that wouldotherwise be filtered prior to final amplification. The anti-peakingsignal is designed not to result in appreciable OOB emissions from itsinteraction with the wanted signal. The amplification system 160includes an anti-peak signal component 166 that generates an anti-peaksignal based on an input signal. The input signal is provided to a firstDAC 162, which converts the input signal from the digital domain to theanalog domain to provide an analog input signal. The anti-peak signal isprovided to a second DAC 168, which converts the anti-peak signal fromthe digital domain to the analog domain to provide an analog anti-peaksignal. The analog anti-peak signal and the analog input signal are thencombined at an analog summer or coupler 164. The combining of theanti-peak signal with the input signal reduces the PAR of the inputsignal provided to the power amplifier 170. This allows the poweramplifier 170 to output a large average power associated withamplification of the peak reduced input signal.

A peak signal is provided to a third DAC 176. The peak signal is asubstantial inversion of the anti-peak signal and removes the anti-peaksignal before final transmission. The third DAC 176 performs adigital-to-analog conversion directly to radio transmission frequencies.The third DAC 176 can be a multi-bit converter or a one-bit converter.The output of the third DAC 176 is then provided to the input terminalof a peak amplifier 178 for amplification. The peak amplifier 178provides an amplified peak signal to a summer or coupler 172 to beaggregated with an analog output signal from the power amplifier 170.The amplified peak signal removes the anti-peak signal from the finaloutput of the amplification system 160. The combined output of thesummer 172 is then provided to an optional band pass filter 174, whichfilters out any remaining unwanted signals outside the desiredtransmission band.

In one aspect of the invention, the added anti-peak signal can be a codechannel that is orthogonal to the wanted signal. It can be in part or inwhole in a separate band from the wanted signals. It may be advantageousto be outside of the passband of the first bandpass or of the finalfilter so that these filters prevent transmission. If it is in thetransmission band, the present invention can cancel it after the finalamplifier but before transmission by a signal sent along the signalcorrection path. If the anti-peaking signal is filtered by a bandpasssignal, it saves dynamic range for the DAC.

FIG. 7 illustrates an amplifier system 200 that employs a second variantof digital cross-cancellation technique in accordance with an aspect ofthe present invention. The amplification system 200 includes a digitalcomponent 202 that receives an input signal and performs a peakreduction of the input signal. The input signal can be in a variety ofdifferent signal formats. For example, the signal can be a signal thatconforms to WCDMA, multi-carrier GSM, OFDM or other signals havingsignatures with high peak-to-average (PAR) ratios.

The digital component 202 can add or remove signals to the input signalto improve the performance of the amplification system 200. For example,the digital component 202 can remove peaks associated with input signal,for example, by clipping and/or adding anti-peak signals to the inputsignal. Additionally, the digital component 202 can performpre-distortion of the composite input signal. Alternatively,pre-distortion can be performed after peak removal or before and/orafter other distortion mitigating techniques. The digital component 202also generates a digital reference signal (REF) associated with thedesired output prior to any modification of the input signal. It is tobe appreciated that the digital reference signal can be a representationof the desired output signal or an inverted representation of thedesired output signal.

The peak reduced input signal is transmitted to a delta-sigma modulator206 along an input path. The delta sigma modulator 206 is coupled to aDAC 208 (e.g., 1-bit DAC, multi-bit DAC), which is coupled to a bandpass filter 210. The delta-sigma modulator 206, the DAC 208 and the bandpass filter 210 cooperate to perform a digital-to-analog conversiondirectly to radio transmission frequencies. The output of the band passfilter 210 is then provided to an optional driver amp 212, whichprovides additional gain to the analog input signal. The output of thedriver 212 is then provided to the input terminal of the power amplifier214 for amplification. The power amplifier 214 can be a linear amplifier(e.g., Class-A, Class-AB, Class-B) or, for some classes of input signal,it can be a non-linear type amplifier (e.g., Class-C, Class-D, Class-E,Class-F) based on desired performance, acceptable efficiency andacceptable OOB emissions.

The digital component 202 provides a reference signal along a correctionpath to a digital phase inverter 224. The reference signal is areference version (REF) of the input signal corresponding to the desiredamplified output signal prior to any modifications. Alternatively, thedigital inverter 224 can be eliminated and the inverted version of theclean reference signal can be provided by the digital component 202. Theinverted reference signal is transmitted to a delta-sigma modulator 226.The delta sigma modulator 226 is coupled to a DAC 228 (e.g., 1-bit DAC,multi-bit DAC) and a band pass filter 230. The delta-sigma modulator226, the DAC 228 and the band pass filter 230 cooperate to perform adigital-to-analog conversion directly to radio transmission frequenciesof the inverted version of the clean reference signal (REF).

A small portion of the power amplifier output is split off through anattenuator 222 and summed with the inverted clean reference signalthrough a summer or coupler 232. The output of the summer 232 is signaldistortion and OOB emissions, including the results of clipping. Theoutput of the summer 232 is amplified by an error amplifier 234 toproduce an error signal (ε). The error signal is inverted through aphase inverter 236 to provide an inverted error signal. The invertederror signal is aggregated with a delayed version of the output of thepower amplifier 214 via a delay component 216 through a summer orcoupler 218 to remove OOB emissions and reduce distortion levels. Theoutput of the summer 218 is then provided to an optional band passfilter 220 that filters out any remaining unwanted signals outside thedesired transmission band. Additional frequency conversion componentscan be utilized in either signal chain, as needed.

The digital cross-cancellation technique in accordance with an aspect ofthe present invention can supply correction for amplifier and othernon-linearities and it can correct spectral splatter that occurs fromintentional clipping of the wanted signals performed to allow foramplifier size reduction. Additionally, since a digital reference signalis employed to determine the desired correction at the output, anymodification of the signal can be corrected at the final output stagewithout the need for additional correction information during theamplification process.

Optionally, a feedback loop through a feedback (FB) path can be providedto sample the output of the bandpass filter 220 of the combined signals,down convert (if needed), and digitize (e.g., with a wideband ADC 204)to examine the entire output transmission band. This optional feedbackloop can be employed in the amplification systems illustrated in FIGS.1–6. The present invention can be employed in other amplifier types suchas an envelope elimination and restoration (EER), envelope trackingamplifier, Doherty amplifier or a Linear Amplification with NonlinearComponents (LINC) amplifier.

It is to be appreciated that the various aspects of the inventionillustrated in FIGS. 1–7 can be employed alone or in a variety ofdifferent combinations. Additionally, the amplification system of thepresent invention can be employed in a number of applications and/orstandards. For example, the amplification system can be employed inwireless transmitter applications for base stations (e.g., satellites,cellular), handsets, and other mobile communication devices.

FIG. 8 illustrates a communication system 250 having a base station 252with a transmitter 260 employing an amplification system 262 inaccordance with an aspect of the present invention. The base station 252employs a central processing unit (CPU) 256 to operate the base station252 and provide an input signal to the amplification system 262. Forexample, the CPU 256 can generate the type of signal (e.g., WCDMA, GSM,OFDM) to be transmitted. The base station 252 communicates to a group ofmobile communication unit (MCUs) comprised of MCUs 286 and 288. The MCUs286 and 288 are for illustrative purposes and it should be appreciatedthat the group of MCUs can include a greater number of MCUs based on thenumber of carriers in the output signal.

The base station 252 also includes cooling devices 254 and power devices258. The power devices 258 can include AC-DC conversion and batterybackup devices that protect the base station 252 from power lossfailures. The power devices 258 and cooling devices 254 can besubstantially reduced in size and cost compared to conventional devicessince the amplification system 262 of the present invention operateswith substantially more efficiency than conventional amplifier systems.Although the base station 252 is illustrated as having a singletransmitter 260, the base station 252 can have a plurality oftransmitters communicating to different respective groups of MCUs oversimilar communication signal standards or different communication signalstandards. Additionally, the MCUs 286 and 288 can also includetransmitters with amplifier systems in accordance with the presentinvention.

The amplification system 262 includes a digital component 264 thatreceives an input signal from the CPU 256 and generates a peak reducesinput signal to a first DAC 266, and a correction signal to a second DAC272. The peak reduced input signal is converted from the digital toanalog domain and provided to an amplifier system 268. The correctionsignal is also converted from the digital to analog domain and providedto the amplifier system 268. The correction signal mitigates signaldistortion and OOB emissions caused by the peak reduction of the inputsignal. The peak reduction and correction can be performed via one ormore techniques as illustrated in the amplification systems of FIGS.1–7. The output of the amplifier system 268 is then provided to anoptional band pass filter 270 that filters out any remaining unwantedsignals outside the desired transmission band. The output of the bandpass filter is then transmitted across a communication link via anantenna 274.

The present invention enables many transmitter architectures to operatewith higher efficiency and substantially smaller part size and cost. Aconventional four-carrier WCDMA system can save 50% or more on the mostcostly part of the transmitter, the final amplifier device. It can alsoimprove its operating efficiency from less than 10% to greater than 20%or more, enabling a significant cost savings in base station capitalequipment.

In view of the foregoing structural and functional features describedabove, methodologies in accordance with various aspects of the presentinvention will be better appreciated with reference to FIGS. 9–10.While, for purposes of simplicity of explanation, the methodologies ofFIGS. 9–10 are shown and described as executing serially, it is to beunderstood and appreciated that the present invention is not limited bythe illustrated order, as some aspects could, in accordance with thepresent invention, occur in different orders and/or concurrently withother aspects from that shown and described herein. Moreover, not allillustrated features may be required to implement a methodology inaccordance with an aspect the present invention.

FIG. 9 illustrates a methodology for amplifying an input signal inaccordance with an aspect of the present invention. The methodologybegins at 300 where peaks associated with an input signal are reduced.The peaks associated with the input signal can be reduced by clippingthe input signal via soft clipping and/or hard clipping employing aclipping filter or the like. The peaks associated with the input signalcan also be reduced by employing a fixed or shaped peak limitingalgorithm. Furthermore, the peaks associated with the input signal canbe reduced by adding an ant-peaking signal to the input signal. One ormore of the above peak reducing techniques can be employed alone or incombination to reduced peaks associated with the input signal. The inputsignal can be an input signal that conforms to a variety of differentwireless formats (e.g., WCDMA, OFDM, multi-carrier versions of GSM, CDMA2000). The methodology then proceeds to 310.

At 310, a correction signal is generated to mitigate one or more of OOBemissions and signal distortion associated with the peak reduction ofthe input signal. At 320, pre-distortion is performed on the peakreduced input signal to mitigate at least a portion of one or more ofthe OOB emissions and signal distortion associated with the peakreduction. At 330, the peak reduced input signal and the correctionsignal are converted from the digital domain to the analog domain, forexample, via separate associated DACs. Separating the paths of thewanted signal and correction signal allows bandwidth and dynamic rangerequirements to be allocated over two or more DACs. The methodology thenproceeds to 340.

At 340, the analog peak reduced input signal and the analog correctionsignal are amplified. At 350, the analog amplified correction signal iscombined with the analog amplified peak reduced input signal to mitigateor cancel OOB emissions and signal distortion caused by peak reductionof the input signal. The correction signal can be combined with the peakreduced input signal via a summer or coupler prior to finalamplification. Alternatively, the correction signal can be amplified bya cancellation amplifier and combined with the peak reduced input signalvia a summer or coupler after amplification of the peak reduced inputsignal by a power amplifier. At 360, the amplified combined outputsignal substantially free of one or more of OOB emissions and signaldistortion is filtered to remove any remaining unwanted signals outsidethe desired transmission band. The final output signal is transmittedover a wireless connection at 370.

FIG. 10 illustrates another methodology for amplifying an input signalin accordance with an aspect of the present invention. The methodologybegins at 400 where peaks associated with an input signal are reduced.The peaks associated with the input signal can be reduced by clippingthe input signal via soft clipping and/or hard clipping, by employing afixed or shaped peak limiting algorithm, and/or adding an ant-peakingsignal to the input signal. The input signal can be an input signal thatconforms to a variety of different wireless formats (e.g., WCDMA, OFDM,multi-carrier versions of GSM, CDMA 2000). The methodology then proceedsto 410. At 410, one or more of the peak reduced input signal, theadjacent spectral channels, and nearby spectral bands are separated intoa plurality of sub-band signals. At 420, associated sub-band signals aremodified, for example, by adjusting the gain, phase and/or offset (allas a function of time or input signal) associated with the individualsub-band signals. The signal modifications are computed to cancel one ormore of the OOB emissions and signal distortion that will be presentafter final amplification. The optional offset term to the conventionalgain and phase modifications, optimizes corrections for each portion ofthe spectrum. The methodology then proceeds to 430.

At 430, the modified sub-band signals are converted from the digitaldomain to the analog domain. At 440, the modified analog sub-bandsignals are aggregated or recombined to provide a recombined analoginput signal. The aggregation may occur prior to conversion to theanalog domain. The aggregated signal will have mitigated OOB emissionsdue to clipping and may contain additional linearization terms. At 450,the aggregated analog input signal is optionally filtered by an acousticwave (SAW) filter or the like. At 460, the aggregated analog inputsignal is then amplified to provide a final amplified output signal fortransmission over a wireless link. One or more additional signals can beadded to the aggregated signal prior to amplification or afteramplification to mitigate any remaining signal distortion and OOBemissions. The amplified signal is passed through an optional bandpassfilter to remove signals outside of the transmission band at 470.

What has been described above includes exemplary implementations of thepresent invention. It is, of course, not possible to describe everyconceivable combination of components or methodologies for purposes ofdescribing the present invention, but one of ordinary skill in the artwill recognize that many further combinations and permutations of thepresent invention are possible. Accordingly, the present invention isintended to embrace all such alterations, modifications and variationsthat fall within the spirit and scope of the appended claims.

1. An amplification system comprising: a channelizer that separates aninput signal into a plurality of sub-bands; a plurality of modificationcomponents that modify at least one of gain, phase and offset associatedwith a corresponding sub-band of the plurality of sub-bands to mitigatedistortion and OOB emissions associated with the sub-band; and anaggregator that recombines the plurality of sub-bands into an aggregatedsignal.
 2. The system of claim 1, further comprising a peak reductioncomponent that reduces peaks associated with an input signal to providea peak reduced input signal.
 3. The system of claim 1, furthercomprising a digital-to-analog converter (DAC) that converts theaggregated signal from the digital domain to the analog domain toprovide an analog aggregated signal and a power amplifier that amplifiesthe analog aggregated signal.
 4. The system of claim 1, furthercomprising a plurality of digital-to-analog converters (DACs) thatconverts the plurality of sub-band signals from the digital domain tothe analog domain prior to recombining the plurality of sub-bands intoan aggregated signal and a power amplifier that amplifies the aggregatessignal.
 5. A method of amplifying an input signal comprising: removingpeaks associated with an input signal to provide a peak reduced inputsignal; separating at least one of a wanted signal, the peak reducedinput signal, adjacent spectral channels, and nearby spectral bands intoa plurality of sub-band signals; modifying at least one of gain, phaseand offset of the at least one sub-band of the plurality of sub-bands tomitigate signal distortion and out-of-band (OOB) emissions; aggregatingthe plurality of sub-bands to provide an aggregated input signal; andamplifying the aggregated input signal to provide a final amplifiedoutput signal.
 6. The method of claim 5, further comprising convertingthe aggregated signal from the digital domain to the analog domain priorto amplifying.
 7. The method of claim 5, further comprising convertingthe plurality of sub-bands from the digital domain to the analog domainprior to aggregation.